The present invention relates to electroluminescent arrangements, a process for their production, as well as their use in particular as distinguishing/identification signs for vehicles. The invention also provides a front diaphragm blank as well as a process for its production.
Electroluminescence (hereinafter also abbreviated to “EL”) is understood to mean the direct excitation of luminescence from luminescent pigments or luminophores by an alternating electric field.
Electroluminescence technology has recently become increasingly important. This technology enables homogeneous luminous surfaces free of dazzle and shadow and of virtually any desired size to be formed. At the same time the power consumption and structural thickness (of the order of magnitude of a millimetre or less) are extremely low. Typical uses include, apart from the background illumination of liquid crystal displays, the back-lighting of transparent films that are provided with lettering and/or image motifs. Thus, transparent electroluminescent arrangements, for example electroluminescent luminous boards based on glass or transparent plastics, which can serve for example as information carriers, advertising panels, or for decorative purposes, are known from the prior art.
A zinc sulfide electroluminescent arrangement based on the use of two electrodes of conducting glass with an electroluminescent phosphor arranged therebetween was already described in 1950 by E. C. Payne in U.S. Pat. No. 2,838,715, and a publication by G. Destriau “The New Phenomenon of Electroluminescence and its Possibilities for the Investigation of Crystal Lattice” in the “Philosophical Magazine” was mentioned by way of reference, in which connection the original discovery of the particular ZnS EL phenomenon in an alternating voltage field was already made by Destriau in 1936.
The luminescent pigments and luminophores that are used in these EL elements are embedded in a transparent, organic or ceramic binder. The starting substances are generally zinc sulfides, which depending on doping or co-doping and preparation procedure generate different, relatively narrow-band emission spectra. The reason for the use of zinc sulfides in the EL layers is due on the one hand to the relatively large number of zinc sulfide EL pigments that are available. The centre of gravity of the spectrum at the same time determines the respective colour of the emitted light. The emission colour of an EL element can be matched by means of a large number of possible measures to the desired colour impression. These measures includes the doping and co-doping of the luminescent pigments, the mixing of two or more EL pigments, the addition of one or more organic and/or inorganic colour-converting and/or colour-filtering pigments, the coating of the EL pigment with organic and/or inorganic colour-converting and/or colour-filtering substances, the admixture of colorants to the polymer matrix in which the luminescent pigments are dispersed, as well as the incorporation of a colour-converting and/or colour-filtering layer or film in the structure of the EL element. In general, depending on the employed doping and co-doping of the zinc sulfide pigments a relatively broad-band emission spectrum is produced if a suitably high alternating voltage of normally greater than 50 volts up to more than 200 volts and a frequency of greater than 50 Hz up to a few kHz, normally in the range from 400 Hz to 2 kHz, is applied.
In order that the produced emission can be seen, at least one flat (planar) electrode is preferably designed to be largely transparent.
Depending on the intended use and production technology, glass substrates or polymeric films with an electrically conducting and largely transparent coating can be used for this purpose. In special embodiments an EL capacitor structure can also be arranged on a substrate in such a way that as front transparent electrode only a thin layer is printed or knife coated, or applied by a roller coating method, a curtain casting method or a spray method. In principle both flat electrodes can also be made largely transparent and in this way a translucent EL element is formed that exhibits a light emission on both sides.
Electroluminescent arrangements are used for example in the field of self-luminescent number plates for vehicles.
Self-luminescent number plates (abbreviation “SLN”), also termed “self-luminescent distinguishing signs” or “self-luminescent distinguishing plates”, are vehicle distinguishing signs that do not have to be illuminated in the dark by an external light source in order that they can be read, but emit light themselves.
At the present time there exist two different types of self-luminescent number plates which are commercially available:                In one type of self-luminescent number plates the lettering is produced by embossing a translucent, white reflecting plastics plate, behind which are arranged white LEDs. For this system, which was developed by the 3M® company, a general model and design approval (ABG) was granted by the German Federal Highways Authority on 10 Jul. 2006.        In a second type of self-luminescent number plates the lettering is printed on a transparent film, which in turn is bonded to an electroluminescent film. The electroluminescent film lights up when an electrical voltage is applied to the film. Number plates according to this principle have likewise been authorised by the German Federal Highways Authority (ABG K55 of 27 Feb. 2007).        
Corresponding electroluminescent systems are also known from the printing technology prior art.
For example, from WO 03/064210 A1 a plate, in particular an identification or number plate for vehicles, with a base body is known, wherein the base body consists of an electrically conducting material or comprises an electrically conducting coating to form a first electrode directly or via a further layer. A coating containing electroluminescent pigmentation is applied to the base body or to the electrically conducting coating, which pigmented coating is in turn covered with an electrically conducting transparent layer to form a second electrode. The base body or the base body with the electroluminescent layer and the electrically conducting transparent layer for forming the second electrode can be shaped, and in particular can be embossed.
In EP 1 463 654 A1 a plate, in particular a vehicle identification plate, is described, which comprises a support made of a plastically deformable material, for example of metal, and an electroluminescent layer structure so as to form at least one flat capacitor. The electroluminescent layer structure has a base electrode, an insulating layer, a pigment layer that luminesces in operation, and a transparent cover electrode. A further insulating layer is applied to the support, above which insulating layer is arranged an electrically conducting layer from which are fashioned the base electrode and at least one power supply lead, electrically separate therefrom, for the cover electrode of the at least one flat capacitor. The further insulating layer is a plastics film that is first of all continuously coated with an electrically conducting material on the side facing away from the support, the said insulating layer having a bracket projecting beyond the support and on which are formed the connection leads necessary for the contacting of the base electrode and the power supply lead.
In EP 0 978 220 A a plastics moulded article with an EL thick-film element is described, wherein this EL thick-film element is three-dimensionally shaped and sprayed on the back with thermoplastic material at an operating temperature below the softening point of the film, and in this way a three-dimensional self-luminescent moulded article is produced.
In German Patent Application DE 10 2006 031 315 of earlier priority date but not prior-published, entitled “3D-EL-HDVF element and production process and use”, a process is described for the production of a three-dimensionally shaped and graphically configured plastics film element consisting of at least one graphically configured plastics film cold-stretchable below the softening point and at least one protective film element, wherein the originally flat and cold-stretchable film with the various cold-stretchable graphical printing together with at least one protective film is moved in an isostatic high-pressure shaping tool, and at a process temperature below the softening point of the plastics film is three-dimensionally shaped in a stress-whitening-free manner with a fluid compression agent at a pressure greater than 20 bar, is at the same time laminated, and is then cut along the edges. In addition the graphical printing is provided with functional properties like an inorganic printable electroluminescent layer sequence.
These distinguishing signs with an electroluminescent effect known from the prior art still have disadvantages in many respects.
Thus, the distinguishing signs with an electroluminescent effect known from the prior art all have the disadvantage that a simple, secure and easily applicable contacting of the distinguishing sign with for example the electrical circuit of the vehicle is not possible. For example, the contacting device described in the aforementioned EP 1 463 654 A and designed as a bracket is complicated to produce. In addition this bracket interferes in the further processing, in particular in the lamination. The cost-effective roll-to-roll lamination process is difficult to execute in this form of implementation, since in this process the bracket is located in the contact region between two endless supports and would have to be machined out in a complicated procedure only later. Furthermore the exposure of the bracket necessitates the provision of diffusion barriers against moisture, since the luminophores are sensitive to moisture and the conducting parts suffer from electrical corrosion under the action of moisture.
Moreover, corresponding distinguishing signs with an electroluminescent effect are often also shaped three-dimensionally, such as for example in the embossing of a distinguishing sign for a vehicle. As a result the electrically conducting coatings can fracture and break, especially in the electrodes. These fractures can mean that electrical conductivity is not ensured over the whole electrode.
Quite apart from this, the cold working property (formability) of corresponding electroluminescent elements is generally not always satisfactorily achieved.
With the automobile distinguishing signs having an electroluminescent effect known from the prior art, in addition the bonding to the substrates is not satisfactorily achieved. Thus, corresponding transitions between the electroluminescent arrangement and substrate in the systems known from the prior art are neither sufficiently stable nor sufficiently durable. Moreover, the transitions do not exhibit adequate shaping properties, which is essential of course especially when embossing automobile distinguishing signs with electroluminescent effects. For example, when embossing an automobile distinguishing sign an embossing of two angles of 90° at a height of 2 mm is generally necessary. In addition the reflecting properties of the automobile distinguishing signs with an electroluminescent property known from the prior art also need to be improved.
Accordingly, the present invention is involved with the object of improving electroluminescent arrangements of the aforementioned type in various respects, especially in the areas identified above as critical.
This object is achieved by the electroluminescent arrangement according to the invention.